Polyamides tend to degrade when exposed to light and/or heat. For example, the amino end groups (i.e., the primary dye sites for acid dyes) of a polyamide are reduced during melt extrusion of the polyamide and during exposure of polyamide fibers or other polyamide articles to light and/or heat. In addition, polyamide fabrics such as, for example, carpets and textiles which have been dyed with certain classes of dyes, lose color and fade when exposed to light and/or heat.
Furthermore, the heat resistance of polyamides such as, for example, nylon 6 and nylon 6/6, is not sufficient for certain applications. For example, during the dyeing of polyamide-containing carpets, yarns, and textile fabrics and in certain heat-setting processes and end-use applications, chemical changes occur which may cause problems, e.g., oxidative/thermal damage. These problems may involve continuous filaments or staple fibers.
Stabilizers have been used to improve the heat resistance of polyamides. The stabilizers may be added before, during, or after polymerization, and even as late as the processing stage. The conventional known stabilizers are admixed with the polymer and are not bound to the polymer chain; therefore, during processing or use of the polyamide, the stabilizers can readily migrate out of the polymer, evaporate, or be washed out. This means that the activity of the stabilization is reduced in an undesired manner, and impurities are released to the surroundings (e.g., air, dye bath, etc.).
Certain copper stabilizers have also been added either during extrusion or in the dye bath to minimize polyamide degradation due to exposure to light and/or heat. This practice, however, is expensive, disturbs processing, and presents environmental problems.
U.S. patent application Ser. No. 09/041,031, filed on Mar. 11, 1998 and now pending which is assigned to BASF Corporation, relates to a process for making stabilized solution-dyed fibers by melting a polyamide comprising amide monomers polymerized in the presence of at least one hindered piperidine compound and coloring the melted polyamide with a colorant.
U.S. patent application Ser. No. 08/804,312, now issued U.S. Pat. No. 5,814,107 on Sep. 29, 1998, which is assigned to BASF Corporation, relates to a process for preparing photochemically-stable, dyed nylon compositions comprising providing to a dye bath a shaped article of poly(epsilon-caprolactam) hydrolytically polymerized in the presence of water and a hindered piperidine derivative and, in the dye bath, dyeing the shaped article with one or more metalized or nonmetalized acid dyestuffs.
A PCT application, International Application No. PCT/EP 95/01349, filed on Apr. 12, 1995 describes an inherently stabilized polyamide containing at least one triacetone diamine compound resented by the formula ##STR1## wherein R is hydrogen, a hydrocarbon group having 1-20 carbon atoms, or a benzyl group. The triacetone diamine compound has a primary amino group (--NH.sub.2) that reacts with a carboxy end group of the polyamide molecule during polymerization, thus rendering the polymer light and heat stable. The available carboxy end groups of the polyamide determine the amount of the one or more triacetone diamine compounds that may be added.
An essay in Poly. Deg. and Stab. 21, 251-262 (1988) describes how the light stability of polyamide 6/6 is improved by adding 2,2,6,6-tetramethylpiperidin-4-ol (TMP). In a recondensation of the TMP-containing polyamide 6/6 in the melt at 275.degree. C. under an atmosphere of water vapor, the authors of the essay claim that TMP reacts with the carboxyl end groups of the polyamides.
Although prior stabilizers are known to be considerably satisfactory, there still remain some problems to be dissolved or improved.